The present invention relates to the general field of so-called “passive” acoustic treatment, and particularly, but not exclusively, in its application to aviation turbine engines.
The invention relates more particularly to a ring of vanes of an aircraft turbine engine, and more particularly to the acoustic treatment of the ring.
The use of passive acoustic treatment panels is frequent in the field of aviation for the purpose of reducing sound emissions from turbine engines. For example, for a turbojet of the two-spool bypass type, such panels may be arranged on the inside face of the nacelle of the turbojet, and also on the inside and outside faces of its primary cover.
A passive acoustic treatment panel is generally made up of a structure forming resonant cavities that is covered by a thin porous layer (also referred to as a perforated skin) acting as a passive wall and closed at its other end by a layer that is reflective from an acoustic point of view (also referred to as a solid skin).
In the context of general specifications concerning noise, which are becoming ever more constraining concerning reducing noise level, and in particular in the context of nacelles that are narrow and short where surface areas available for potential acoustic treatment are becoming smaller and smaller, it is difficult to satisfy the general specifications in conventional manner. Specifically, there is less room for installing the equipment, in particular acoustic panels, that is used for attenuating noise from the fan. Thus, the volume of the equipment becomes a major problem, as does integrating it, in particular installing acoustic panels in the secondary passage of the engine.
Furthermore, in a ring of vanes, also known as outlet guide vanes (OGV), the surfaces of the air flow passage are built up from auxiliary parts such as platforms mounted between the outlet guide vanes so as to be interposed close to the roots and close to the tips of those vanes. Generally, acoustic treatment panels are used immediately upstream and downstream from the rings of outlet guide vanes. The ring itself generally does not have acoustic treatment, thereby reducing the area of the acoustic treatment and also reducing the effectiveness of the acoustic treatment.
Such rigid platforms without acoustic treatment, and thus forming a zone of acoustic leakage, can also be excited by the secondary stream, thereby adding to design constraints.
The platforms forming the inner and outer shrouds of the ring and making up the surfaces of the air flow passage through the ring of outlet guide vanes generally include a large amount of ribbing in order to strengthen the platforms and reduce any risk of the integrity of the ring structure being degraded.
Rings of vanes for a turbojet are known that include sound attenuation panels with resonant cavities, the panels being inserted in recesses made in platform portions and in vane portions.
Nevertheless, those panels are capable of covering only a fraction of the surface of the shroud made up of the platforms of the vanes, which shroud may be thought of as an annular sheath. Specifically, in order to conserve the integrity of the structure of the ring, a recess can be made in only a portion of the platform.